1. Field of the Invention
This invention relates to a method and system for acquiring CT data by using continuous radiation in a CT apparatus.
2. Description of the Related Art
In the case of a conventional CT (computerized tomography) apparatus, an X-ray source generating a fan beam and an X-ray detector having channels opposite to the X-ray source are rotated in unison around a subject under examination so as to acquire X-ray data transmitted through the object. The following are methods of acquiring X-ray data from one fan beam:
(1) The X-ray data is simultaneously acquired by all the channels of the X-ray detector, and the data acquired by the respective channels is transferred, in sequential fashion, to a data processor.
(2) The channels of the X-ray detector are operated on a time-division basis to sequentially acquire the X-ray data, the X-ray data acquired by each channel being transferred to the data processor immediately upon acquisition.
In the case of the first method, when the X-ray data is acquired by continuous emission of X-rays, an additional period of time from the completion of data acquisition until the completion of data transfer is required for all the channels of the detector. Moreover, since the continuous X-ray is used, X-ray transmitted during the additional period of time becomes useless.
With the second method, on the other hand, since the timing of X-ray data acquisition varies with the channels of the detector, the following problems will arise.
For example, as is shown in FIG. 1A, when X-ray source 1 and X-ray detector 4, which has channels opposite to source 1, are rotated in direction 3a, with the center 2 of object P as the axis of rotation--in other words, while X-ray source 1 generating an X-ray fan beam is moved from a position 1a to a position 1b--X-ray detector 4 acquires X-ray data, in sequential fashion, from the data of first channel 4b, corresponding to X-ray pass 7, to the data of 512th channel 4a, corresponding to X-ray pass 8a, along the data acquiring direction 6 (refer to FIG. 2). This corresponds to the case where data is acquired on the basis of a fan beam transmitted from an intersectional point 9 (imaginary point) of the X-ray passes, the position of such an imaginary point 9 being closer to object P than the actual position 1a or 1b of source 1.
When, on the other hand, source 1 is rotated in the opposite direction 3b, as is shown in FIG. 1B, data is acquired sequentially along direction 6, from first channel 4b corresponding to X-ray pass 7 of source 1 at position 1a, to 512th channel 4a corresponding to X-ray pass 8b of source 1 at position 1c. In this case, intersectional point 10 (the imaginary point of this case) of X-ray passes 7 and 8b is located outside the actual position 1a or 1c of source 1 with respect to object P.
As described above, when the data acquiring direction is constant regardless of the rotational direction of the source, data based on different scaling factors will be acquired because the position of the imaginary source varies by directions of rotation of the actual source. Thus, in reconstructing a CT image, various parameters which would be affected by different scaling factors must be adjusted so as to correct the data obtained. When data correction is performed by computer processing, however, problems will arise in the processing period of time, the memory capacity for data, etc.
Therefore, it is desired to acquire CT data so as to reconstruct CT images without data correction due to different scaling factors.